KIEV, Ukraine — Warning that it was “on the brink of disaster,” Ukraine put its military on high alert Sunday and appealed for international help to avoid what it feared was the possibility of a wider invasion by Russia.

Outrage over Russia’s military moves mounted in world capitals, with U.S. Secretary of State John Kerry calling on President Vladimir Putin to pull back from “an incredible act of aggression.”

A day after Russia captured the Crimean Peninsula without firing a shot, fears grew in the Ukrainian capital and beyond that Russia might seek to expand its control by seizing other parts of eastern Ukraine. Senior Obama administration officials said the U.S. now believes Russia has complete operational control of Crimea, a pro-Russian area of the country, and has more than 6,000 troops in the region.

World leaders rushed to try to find a diplomatic solution to the crisis.

NATO held an emergency meeting in Brussels, Britain’s foreign minister flew to Kiev to support its new government and Kerry was to travel to Ukraine on Tuesday. The U.S., France and Britain debated the possibility of boycotting the next Group of Eight economic summit, to be held in June in Sochi, the host of Russia’s successful Winter Olympics.

In Kiev, Moscow and other cities, thousands of protesters took to the streets either to decry the Russian occupation or to celebrate Crimea’s return to its former ruler.

“Support us, America!” a group of protesters chanted outside the U.S. Embassy in Kiev. One young girl held up a placard reading: “No Russian aggression!”

“Russia! Russia!” the crowd chanted in Moscow.

Kerry, interviewed on U.S. television news shows, talked about boycotting the G-8 summit, as well as possible visa bans, asset freezes and trade and investment penalties against Russia. All the foreign ministers he talked to were prepared “to go to the hilt” to isolate Russia, Kerry said.

President Barack Obama also spoke Sunday with German Chancellor Angela Merkel, British Prime Minister David Cameron and Polish President Bronislaw Komorowski.

NATO issued a statement saying it “condemns Russia’s military escalation in Crimea” and demanded that Russia respect its obligations under the U.N. charter. Ukraine is not a NATO member, meaning the U.S. and Europe are not obligated to come to its defense, but the country has taken part in some alliance exercises.

“We are on a very dangerous track,” German Foreign Minister Frank-Walter Steinmeier said. But “it is still possible to turn around. A new division of Europe can still be prevented.”

So far, however, Ukraine’s new government and the West have been powerless to counter Russia’s tactics. Armed men in uniforms without insignia have moved freely about Crimea for days, occupying airports, smashing equipment at an air base and besieging a Ukrainian infantry base.

Putin has defied calls from the West to pull back his troops, insisting that Russia has a right to protect its interests and those of Russian-speakers in Crimea and elsewhere in Ukraine. His confidence is matched by the knowledge that Ukraine’s 46 million people have divided loyalties. While much of western Ukraine wants closer ties with the 28-nation European Union, its eastern and southern regions like Crimea look to Russia for support.

Russia has long wanted to reclaim the Crimean Peninsula, part of its territory until 1954. Russia’s Black Sea Fleet pays Ukraine millions annually to be stationed at the Crimean port of Sevastopol and nearly 60 percent of Crimea’s residents identify themselves as Russian.

During a phone conversation Sunday with Merkel, Putin “directed her attention to the unrelenting threat of violence from ultranationalist forces (in Ukraine) that endangered the life and legal interests of Russian citizens,” according to a Kremlin statement.

“The measures taken by Russia are fully adequate with regard to the current extraordinary situation,” it said.

The German government said Putin had accepted a proposal by Merkel to set up a “contact group” aimed at facilitating dialogue in the Ukraine crisis.

Russia’s state-controlled media has played almost nonstop footage of the Ukrainian crisis, highlighting what it says are ultranationalist attacks on Russians and pro-Russian Ukrainians by activists from Kiev or regions further west. However, AP reporters in Ukraine witnessed no acts of violence directed at Russians or Russian sympathizers in Crimea.

Ukraine’s richest man, Rinat Akhmetov, urged business, ordinary people and the government to join together, saying Sunday that the use of force and “illegal action from outside” were “impermissible.”

“I call upon all my fellow citizens to unity for the sake of a whole and undivided Ukraine … Our strength is in the solidarity of business, government and society,” said Akhmetov, whose SCM Group has 300,000 employees and interests in steel, coal and mining.

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